Third generation (3G) mobile systems, such as, for instance, universal mobile telecommunication systems (UMTS) standardized within the third generation partnership project (3GPP) have been based on wideband code division multiple access (WCDMA) radio access technology. Today, 3G systems are being deployed on a broad scale all around the world. After enhancing this technology by introducing high-speed downlink packet access (HSDPA) and an enhanced uplink, also referred to as high-speed uplink packet access (HSUPA), the next major step in evolution of the UMTS standard has brought the combination of orthogonal frequency division multiplexing (OFDM) for the downlink and single carrier frequency division multiplexing access (SC-FDMA) for the uplink. This system has been named long term evolution (LTE) since it has been intended to cope with future technology evolutions.
The LTE system represents efficient packet based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in 3GPP TR 25.913, “Requirements for evolved UTRA (E-UTRA) and evolved UTRAN (E-UTRAN),” v8.0.0, January 2009 (incorporated herein by reference). The Downlink will support data modulation schemes QPSK, 16QAM, and 64QAM and the Uplink will support BPSK, QPSK, 8PSK and 16QAM.
LTE's network access is to be extremely flexible, using a number of defined channel bandwidths between 1.25 and 20 MHz, contrasted with UMTS terrestrial radio access (UTRA) fixed 5 MHz channels. Spectral efficiency is increased by up to four-fold compared with UTRA, and improvements in architecture and signalling reduce round-trip latency. Multiple Input/Multiple Output (MIMO) antenna technology should enable 10 times as many users per cell as 3GPP's original WCDMA radio access technology. To suit as many frequency band allocation arrangements as possible, both paired (frequency division duplex FDD) and unpaired (time division duplex TDD) band operation is supported. LTE can co-exist with earlier 3GPP radio technologies, even in adjacent channels, and calls can be handed over to and from all 3GPP's previous radio access technologies.
FIG. 1 illustrates structure of a component carrier in LTE Release 8. The downlink component carrier of the 3GPP LTE Release 8 is sub-divided in the time-frequency domain in so-called sub-frames 100 each of which is divided into two downlink slots, one of which is shown in FIG. 1 as 120 corresponding to a time period Tslot. The first downlink slot comprises a control channel region within the first OFDM symbol(s). Each sub-frame consists of a given number of OFDM symbols in the time domain, each OFDM symbol spanning over the entire bandwidth of the component carrier.
In particular, the smallest unit of resources that can be assigned by a scheduler is a resource block also called physical resource block (PRB). A PRB 130 is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive sub-carriers in the frequency domain. In practice, the downlink resources are assigned in resource block pairs. A resource block pair consists of two resource blocks. It spans NscRB consecutive sub-carriers in the frequency domain and the entire 2 NsymbDL modulation symbols of the sub-frame in the time domain. NsymbDL may be either 6 or 7 resulting in either 12 or 14 OFDM symbols in total. Consequently, a physical resource block 130 consists of NsymbDL×NscRB resource elements corresponding to one slot in the time domain and 180 kHz in the frequency domain (further details on the downlink resource grid can be found, for example, in 3GPP TS 36.211, “Evolved universal terrestrial radio access (E-UTRA); physical channels and modulations (Release 8)”, version 8.9.0, December 2009, Section 6.2, available at http://www.3gpp.org. which is incorporated herein by reference).
The number of physical resource blocks NRBDL in downlink depends on the downlink transmission bandwidth configured in the cell and is at present defined in LTE as being from the interval of 6 to 110 PRBs.
The data are mapped onto physical resource blocks by means of pairs of virtual resource blocks. A pair of virtual resource blocks is mapped onto a pair of physical resource blocks. The following two types of virtual resource blocks are defined according to their mapping on the physical resource blocks in LTE downlink:                Localised Virtual Resource Block (LVRB)        Distributed Virtual Resource Block (DVRB)        
In the localised transmission mode using the localised VRBs, the eNB has full control which and how many resource blocks are used, and should use this control usually to pick resource blocks that result in a large spectral efficiency. In most mobile communication systems, this results in adjacent physical resource blocks or multiple clusters of adjacent physical resource blocks for the transmission to a single user equipment, because the radio channel is coherent in the frequency domain, implying that if one physical resource block offers a large spectral efficiency, then it is very likely that an adjacent physical resource block offers a similarly large spectral efficiency. In the distributed transmission mode using the distributed VRBs, the physical resource blocks carrying data for the same UE are distributed across the frequency band in order to hit at least some physical resource blocks that offer a sufficiently large spectral efficiency, thereby obtaining frequency diversity.
In 3GPP LTE Release 8 there is only one component carrier in uplink and downlink. Within one DL subframe, the first 1 to 4 OFDM symbols are used for downlink control channel and downlink signal transmission (LTE control region). FIG. 5 shows an example of LTE DL control region. Downlink control signalling is basically carried by the following three physical channels:                Physical control format indicator channel (PCFICH) for indicating the number of OFDM symbols used for control signalling in a sub-frame (i.e. the size of the control channel region). For NRBDL>10, the PCFICH value is between 0 and 3.        Physical hybrid ARQ indicator channel (PHICH) for carrying the downlink ACK/NACK associated with uplink data transmission. The duration of PHICH, i.e. the number of OFDM symbols used for PHICH, is configured by higher layer. For normal PHICH, the duration is 1 OFDM symbol. For extended PHICH, the duration is 2 to 3 OFDM symbols. The duration of PHICH puts a lower limit on the size of the DL control region determined from the PCFICH value.        Cell-specific reference signals (CRS) are transmitted on one or several of antenna ports 0 to 3. In a normal subframe, CRS is distributed within the subframe across the whole bandwidth. In an MBSFN subframe, CRS shall only be transmitted in the non-MBSFN region, i.e. DL control region, of the MBSFN subframe.        Physical downlink control channel (PDCCH) for carrying downlink scheduling assignments and uplink scheduling assignments.        
The PCFICH is sent from a known position within the control signalling region of a downlink sub-frame using a known pre-defined modulation and coding scheme. The user equipment decodes the PCFICH in order to obtain information about a size of the control signalling region in a sub-frame, for instance, the number of OFDM symbols. If the user equipment (UE) is unable to decode the PCFICH or if it obtains an erroneous PCFICH value, it will not be able to correctly decode the L1/L2 control signalling (PDCCH) comprised in the control signalling region, which may result in losing all resource assignments contained therein.
The PDCCH carries control information, such as, for instance, scheduling grants for allocating resources for downlink or uplink data transmission. A physical control channel is transmitted on an aggregation of one or several consecutive control channel elements (CCEs). Each CCE corresponds to a set of resource elements grouped to so-called resource element groups (REG). A control channel element typically corresponds to 9 resource element groups. A scheduling grant on PDCCH is defined based on control channel elements (CCE). Resource element groups are used for defining the mapping of control channels to resource elements. Each REG consists of four consecutive resource elements excluding reference signals within the same OFDM symbol. REGs exist in the first one to four OFDM symbols within one sub-frame. The PDCCH for the user equipment is transmitted on the first of either one, two or three OFDM symbols according to PCFICH within a sub-frame.
Another logical unit used in mapping of data onto physical resources in 3GPP LTE Release 8 (and later releases) is a resource block group (RBG). A resource block group is a set of consecutive (in frequency) physical resource blocks. The concept of RBG provides a possibility of addressing particular RBGs for the purpose of indicating a position of resources allocated for a receiving node (e.g. UE), in order to minimise the overhead for such an indication, thereby decreasing the control overhead to data ratio for a transmission. The size of RBG is currently specified to be 1, 2, 3, or 4, depending on the system bandwidth, in particular, on NRBDL. Further details of RBG mapping for PDCCH in LTE Release 8 may be found in 3GPP TS 36.213 “Evolved Universal terrestrial Radio Access (E-UTRA); Physical layer procedures”, v8.8.0, September 2009, Section 7.1.6.1, freely available at http://www.3gpp.org/ and incorporated herein by reference.
The UE shall monitor a set of PDCCH candidates on the serving cell for control information in every non-DRX subframe, where monitoring implies attempting to decode each of the PDCCHs in the set according to all the monitored DCI formats. The set of PDCCH candidates to monitor are defined in terms of search spaces.
UE monitors two types of search space: UE specific search space and common search space. Both UE specific search space and common search space have different aggregation levels.
In UE specific search space, there are {6,6,2,2} number of PDCCH candidates at the respective aggregation levels {1,2,4,8} and the PDCCH candidates of each aggregation level are consecutive in CCEs. The starting CCE index of the first PDCCH candidate in aggregation level L is decided by Yk×L, wherein k is the subframe number and Yk is decided by k and UE ID. Therefore, the positions of CCEs in UE specific search space are decided by UE ID to reduce the overlapping of PDCCH UE specific search space from different UEs and are randomized from subframe to subframe to randomized the interference from PDCCH in neighboring cells.
In the common search space, there are {4,2} number of PDCCH candidates at respective aggregation levels {4,8}. The first PDCCH candidate in aggregation level L starts from CCE index 0. Therefore, all the UEs monitor the same common search space.
PDCCH for system information is transmitted in common search space, so that all the UEs can receive system information by monitoring common search space.
Physical downlink shared channel (PDSCH) is used to transport user data. PDSCH is mapped to the remaining OFDM symbols within one sub-frame after PDCCH. The PDSCH resources allocated for one UE are in the units of resource block for each sub-frame. In LTE, DL data region starts after the DL control region within one subframe. In DL data region, CRS, PDSCH and corresponding DM-RS are transmitted.
FIG. 2 shows an exemplary mapping of PDCCH and PDSCH within a sub-frame. The first two OFDM symbols form a control channel region (PDCCH region) and are used for L1/L2 control signalling. The remaining twelve OFDM symbols form data channel region (PDSCH region) and are used for data. Within a resource block pairs of all sub-frames, cell-specific reference signals, so-called common reference signals (CRS), are transmitted on one or several antenna ports 0 to 3. In the example of FIG. 3, the CRS are transmitted from two antenna ports: R0 and R1.
Moreover, the sub-frame also includes UE-specific reference signals, so-called demodulation reference signals (DM-RS) used by the user equipment for demodulating the PDSCH. The DM-RS are only transmitted within the resource blocks in which the PDSCH is allocated for a certain user equipment. In order to support multiple input/multiple output (MIMO) with DM-RS, four DM-RS layers are defined meaning that at most, MIMO of four layers is supported. In this example, in FIG. 3, DM-RS layer 1, 2, 3 and 4 are corresponding to MIMO layer 1, 2, 3 and 4.
In September 2009 the 3GPP Partners made a formal submission to the ITU proposing that LTE Release 10 & beyond (LTE-Advanced) be evaluated as a candidate for IMT-Advanced. The ITU has coined the term IMT Advanced to identify mobile systems whose capabilities go beyond those of IMT 2000. In order to meet this new challenge, 3GPPs Organizational Partners have agreed to widen 3GPP's scope to include systems beyond 3G. In 3GPP, further advancements for E-UTRA (LTE-Advanced) should be studied in accordance with: 3GPP operator requirements for the evolution of E-UTRA and the need to meet/exceed the IMT-Advanced capabilities. The expectancy is that Advanced E-UTRA should provide substantially higher performance compared to what is expected to be the IMT-Advanced requirements in ITU-R.
LTE-A Rel.10 work started from March 2010 and has already stable in June 2011. The major features included in LTE-A Rel.10 included Carrier Aggregation, enhanced DL MIMO, UL MIMO, relay and etc.
According to 3GPP TS 36.300 v.2.0, in Carrier Aggregation (CA), two or more Component Carriers (CCs) are aggregated in order to support wider transmission bandwidths up to 100 MHz. A UR may simultaneously receive or transmit on one or multiple CCs depending on its capabilities. It is possible to configure a UE to aggregate a different number of CCs in the UL and the DL.                The number of DL CCs that can be configured depends on the DL aggregation capability of the UE;        The number of UL CCs that can be configured depends on the UL aggregation capability of the UE;        It is not possible to configure a UE with more UL CCs than DL CCs.        
When CA is configured, a UE only has one RRC connection with the network. At RRC connection reestablishment/handover, one serving cell provides the NAS mobility information and the security input. The serving cell is referred as Primary Cell (PCell). In the downlink, the carrier corresponding to the PCell is Downlink Primary Component Carrier (DL PCC) while in the uplink it is Uplink Primary Component Carrier (UL PCC).
When CA is configured, a UE may be scheduled over multiple serving cells simultaneously. On each DL CC, there is DL control region. PDSCH on one DL CC can be scheduled by the PDCCH on this DL CC or it can also be cross-carrier scheduled by the PDCCH on another DL CC. Carrier Indicator Field (CIF) allows the PDCCH of a serving cell to schedule resources on another serving cell but with the following restrictions:                Cross-carrier scheduling does not apply to PCell, i.e. PCell is always scheduled via its PDCCH;        When the PDCCH of an SCell is configured, cross-carrier scheduling does not apply to this SCell i.e. it is always scheduled via its PDCCH;        When the PDCCH of an SCell is not configured, cross-carrier scheduling applies and this SCell is always scheduled via the PDCCH of one other serving cell.        
In LTE-A, DL control region of LTE is reused. PCFICH is used to indicate the size of DL control region. PHICH carries the hybrid-ARQ ACK/NACK. PDCCH search space is extended to support cross-carrier scheduling.
In LTE-A, UE shall monitor PDCCH search space in one or more activated serving cells as configured by higher layer. For each serving cell on which PDCCH shall be monitored, UE shall monitor UE specific search space of the given serving cell and also UE specific search spaces of the serving cells that are cross-carrier scheduled on the given serving cell.
In LTE-A, DL data region also starts after the DL control region within one subframe. DMRS is used for PDSCH demodulation. Up to 8 layers DM-RS are supported.
Another key feature of the LTE-A is providing relaying functionality by means of introducing relay nodes to the UTRAN architecture of 3GPP LTE-A. Relaying is considered for LTE-A as a tool for improving the coverage of high data rates, group mobility, temporary network deployment, the cell edge throughput and/or to provide coverage in new areas. A relay node is wirelessly connected to radio access network via a donor cell. Depending on the relaying strategy, a relay node may be part of the donor cell or, alternatively, may control the cells on its own. In case the relay node is a part of the donor cell, the relay node does not have a cell identity on its own, however, may still have a relay ID. In the case the relay node controls cells on its own, it controls one or several cells and a unique physical layer cell identity is provided in each of the cells controlled by the relay.
FIG. 4 illustrates an example 3GPP LTE-A network structure using relay nodes. A donor eNodeB (d-eNB) 410 directly serves a user equipment UE1 415 and a relay node (RN) 420 which further serves UE2 425. The link between donor eNodeB 410 and the relay node 420 is typically referred to as relay backhaul uplink/downlink. The link between the relay node 420 and user equipment 425 attached to the relay node (also denoted r-UEs) is called (relay) access link. The donor eNodeB transmits L1/L2 control and data to the micro-user equipment UE1 415 and also to a relay node 420 which further transmits the L1/L2 control and data to the relay-user equipment UE2 425. The relay node may operate in a so-called time multiplexing mode, in which transmission and reception operation cannot be performed at the same time. In particular, if the link from eNodeB 410 to relay node 420 operates in the same frequency spectrum as the link from relay node 420 to UE2 425, due to the relay transmitter causing interference to its own receiver, simultaneous eNodeB-to-relay node and relay node-to-UE transmissions on the same frequency resources may not be possible unless sufficient isolation of the outgoing and incoming signals is provided. Thus, when relay node 420 transmits to donor eNodeB 410, it cannot, at the same time, receive from UEs 425 attached to the relay node. Similarly, when a relay node 520 receives data from donor eNodeB, it cannot transmit data to UEs 425 attached to the relay node. Thus, there is a sub-frame partitioning between relay backhaul link and relay access link.
In relaying operation, a new PDCCH, i.e. relay physical downlink control channel (R-PDCCH) is defined to carry DCI for relay nodes. R-PDCCH is transmitted in the LTE-A data region. PDCCH for DL assignment starts from the 4th OFDM symbol in 1st slot and ends at the last OFDM symbol in 1st slot; PDCCH for UL grant starts from the first OFDM symbol in 2nd slot and ends at the last or the one before last OFDM symbol. There is no overlap between Rel. 8-10 control region and R-PDCCH region, since relay does not need to receive Rel.8-10 PDCCH from eNB after it was connected to eNB.
In the frequency domain, a set of PRBs is configured for potential R-PDCCH transmission by higher layers.
An R-PDCCH can be transmitted on one or several PRBs without being cross-interleaved with other R-PDCCHs in a given PRB. Alternatively, multiple R-PDCCHs can be cross-interleaved in one or several PRBs.
The R-PDCCH without cross-interleaving shall be demodulated based on cell-specific reference signals transmitted on one set of antenna ports {0}, {01}, or {0,1,2,3}, or based on UE-specific reference signals transmitted on antenna port 7 assuming that nSCID=0; the type of reference signals is configured by higher layers. Spatial multiplexing is not supported for R-PDCCH.
In 3GPP LTE, the resources may be allocated in terms of physical resource blocks (PRB). Some control channels allow for assigning even smaller resource portions. For instance, the PDCCH control channel region within a sub-frame consists of a set of control channel elements (CCEs). A PDCCH can aggregate 1, 2, 4 or 8 CCEs. Similarly, R-PDCCH shall likely support aggregation levels 1, 2, 4, and 8. The aggregation may be over CCEs or over physical resource blocks.
Each relay node monitors a set of R-PDCCH candidates of any aggregation levels for control information in every non-DRX subframe. Monitoring refers to attempting to decode each of the R-PDCCHs in the set according to all monitored formats, i.e. blind decoding. Blind decoding is described for UE receiving a PDCCH in 3GPP TS 36.213 “Evolved Universal terrestrial Radio Access (E-UTRA); Physical layer procedures”, v8.8.0, September 2009, Section 9.1.1, freely available at http://www.3gpp.org/ and incorporated herein by reference). According to the present specifications for UE-specific PDCCH, the search space may include six candidates of aggregation level 1 and 2 and two candidates of aggregation levels 4 and 8. The number of candidates also specifies the number of blind decodings the terminal has to perform.
The search space configuration in terms of resources available to carry R-PDCCH may be configured semi-statically (for instance, by RRC) or fixed.
LTE-A Release 11 work started from September 2011. The major features of LTE-A Release 11 include LTE carrier aggregation enhancements, Further Enhanced Non CA-based ICIC (inter-cell interference coordination) for LTE, Coordinated Multi-Point Operation (CoMP) for LTE—Downlink and etc. Besides, LTE-A Release 11 also includes studies on Coordinated Multi-Point operation (CoMP) for LTE, Enhanced Uplink Transmission for LTE, further Downlink MIMO enhancements for LTE-Advanced.
During the study on CA enhancement, CoMP and DL MIMO, current PDCCH defined in Releases 8-10 shows some disadvantages: Beamforming or spatial multiplexing is not possible, frequency scheduling gain with localized allocation is not possible, because of only distributed DCI transmission is supported and frequency ICIC (Inter-Cell Interference Coordination) is not possible, because of random REG allocation among cells. In order to improve the situation, an enhanced PDCCH (E-PDCCH) is worked on, which would avoid the above problems.
Some contributions discussed within the 3GPP already address the design of E-PDCCH. One of the solutions suggested is to reuse R-PDCCH, extending the design to support spatial multiplexing of E-PDCCH. However, R-PDCCH always starts from the 4th OFDM symbol and is mapped into the data channel area. This solution may lead to resource wasting in case of Release-11 terminals which may leave large portions of UE-specific PDCCH unused.
Another solution (Contribution number R1-113322) suggests inserting new DM-RS into E-PDCCH control channel elements in order to support beamforming and spatial multiplexing. Accordingly, E-PDCCH CCEs are also interleaved with PDCCH CCEs and are distributed across the entire bandwidth. In MBSFN subframes, E-PDCCH is transmitted after PDCCH region and, again, distributed across the entire bandwidth. With this configuration, E-PDCCH is distributed over the whole bandwidth. Thus, frequency-domain ICIC and frequency-selective scheduling cannot be supported.